The present invention relates to a Doppler-type diagnostic apparatus, and more particularly to the apparatus that is in use for diagnosing motion of fluids such as blood within an object being examined by utilizing the Doppler effect of ultrasonic signals.
At present, there has already been provided a Doppler-type ultrasonic diagnostic apparatus in which ultrasonic pulse Doppler and ultrasonic pulse reflection methods are used together to obtain a tomographic image (black/white B-mode image) and a real-time blood flow image through a single ultrasonic probe.
FIG. 1 exemplifies such a Doppler-type ultrasonic diagnostic apparatus by which the speed of a blood flow is measured as blood flow information. In the apparatus shown in the figure, connected to an ultrasonic probe 201 are a transmitting pulser 202 and receiving pre-amplifier 203. The output of the pre-amplifier 203 is connected, by way of a mixer 204, lowpass filter 205, sample and hold circuit 206, bandpass filter 207, and frequency analyzer 208, in turn, with a display unit 209.
Further, the diagnostic apparatus is also provided with a pulse generator 210 for transmitting/receiving control and a range gate circuit 211 for range gate control. The pulse generator 210 incorporates frequency dividers and gate circuits, thus supplying a clock pulse S.sub.a of a specific frequency (refer to FIG. 2) to the range gate circuit 211 and mixer 204, creating a rate pulse S.sub.b of ultrasonic repetition frequency (refer to FIG. 2) on the basis of the clock pulse S.sub.a and supplying it to the pulser 202 and range gate circuit 211.
The pulser 202 creates a high-voltage driving pulse using the supplied rate pulse S.sub.b in order to drive the ultrasonic probe 201. This driving allows the probe 201 to transmit an ultrasonic pulse signal into an object P. Part of the transmitted ultrasonic signal makes an ultrasonic echo signal by being reflected at the wall BW and blood flow B (mainly, red corpuscle) of a blood vessel BV. The ultrasonic echo signal will then be received by the probe 201, where a corresponding electrical echo signal S.sub.d (refer to FIG. 2) is yielded.
The electrical echo signal S.sub.d reflects the Doppler effect, that is, the Doppler shift in frequency caused by scattering of the ultrasonic signal by corpuscles in motion. According to this effect, the central frequency f.sub.c of a received ultrasonic echo signal changes by a Doppler shift frequency f.sub.d, thus making its receiving frequency f=f.sub.c +f.sub.d. The Doppler shift frequency f.sub.d is approximately expressed as follows by assuming a blood flow speed is v, an angle between an ultrasonic beam and a blood vessel is .theta., and a sound speed is c. EQU f.sub.d ={(2.multidot.v.multidot.cos .theta.19 f.sub.c)/c}.multidot.f.sub.c
Detecting the Doppler shift frequency fd from the received electrical echo signal Sd provides the blood flow speed v, which gives a foundation to the receiving system of the present Doppler-type ultrasonic diagnostic apparatus. In detail, the electrical echo signal S.sub.d is amplified by the pre-amplifier 203 and then sent to the mixer 204, where the amplified echo signal S.sub.d is mixed with the clock pulse S.sub.a to be supplied to the next lowpass filter 205. The mixed signal is lowpassed by the lowpass filter 205, so that higher-frequency components such as an ultrasonic carrier are removed from the mixed signal; only low frequency components centered on the Doppler shift frequency f.sub.d are sent to the sample and hold circuit 206.
By using a sampling pulse S.sub.c (refer to FIG. 2) that corresponds to the distance from the surface of an object to the position O of a range gate (i.e., sampling point or sampling volume) placed on a blood flow B of a tomographic image, the filtered signal is then sampled and held by the sample and hold circuit 206, the held signal being sent to the bandpass filter 207 by which excessive higher and lower frequency components are removed to extract only the Doppler shift frequency component of the blood flow B. The extracted signal is then frequency-analyzed with fast Fourier transformation, for example, for obtaining a frequency spectrum pattern of Doppler shift frequencies (i.e., Doppler spectrum). This Doppler spectrum, which is displayed on the display unit 209 as shown in FIG. 3, represents changes in Doppler shift frequency in a two-dimensional coordinate system whose vertical axis is assigned to the frequency and its horizontal axis to time, where strength of each frequency component is depicted by altering pixel brightness.
For a further examination of blood flows, it is sometimes required to observe changes in time of maximum speeds of blood flows. In such observation, the maximum frequencies of a real-time Doppler spectrum are automatically traced on its image or the maximum frequencies of a frozen Doppler spectrum are traced by hand, as shown by a bold line MF in FIG. 4, so that the traced values (maximum frequencies) are extracted.
Another analysis technique is to use a histogram of flow speeds, which is a speed component distribution where its horizontal axis is assigned to frequencies (i.e., Doppler shift frequencies corresponding to blood flow speeds) and its vertical axis is assigned to powers (strength) of respective frequencies. To obtain the histogram of blood flows, a Doppler spectrum image is first frozen and a desired time position in the horizontal axis of the spectrum image is then specified with a cursor marker (for instance, refer to a marker MT in FIG. 5). In response to this, a calculator (not shown) works to calculate a distribution of speed components at the specified time position. The calculated distribution data is normally displayed as shown in FIG. 6.
However, there are a wide variety of drawbacks in the above image processing. All of those drawbacks are resulted from the fact that the ultrasonic echo signal received by the probe 201 includes known speckle components caused by phase interferences of reflected echoes of corpuscles in a blood flow.
Concretely, the speckle components makes a Doppler spectrum change little by little, but rough, in the vertical frequency (blood flow) direction, as shown in FIG. 3, and a histogram of flow speeds changes largely and narrowly in the vertical power axis, as shown in FIG. 6, thus deteriorating accuracy in displaying a blood flow.
As a result, when a Doppler spectrum is observed, it is difficult to recognize a steady frequency range at a glance, because there are large changes in the frequency and density.
In addition, automatic or manual tracing the maximum frequencies on a Doppler spectrum results in small and frequent ups and downs of a traced curve, which makes the tracing difficult and inefficient and imposes comparatively heavy burden on an operator.
Further, when such a histogram of flow speeds shown in FIG. 6 is observed, it is hard to recognize how the entire histogram image spreads. In this case, it is necessary to repeat the specification of another time position on a Doppler histogram for displaying revised histogram images. This operation necessarily involves frequently-repeated judgement of whether or not a histogram image now on is an observable distribution images. Therefore, in case of obtaining a histogram of flow speeds by the conventional technique, much operation load will be put on an operator and a diagnostic time will be longer.